Two-factor authentication

Two-factor authentication (TFA or 2FA) is an approach to authentication which requires the presentation of two different kinds of evidence that someone is who they say they are. It is a part of the broader family of multi-factor authentication, which is a defense in depth approach to security. From a security perspective, the idea is to use evidences which have separate range of attack vectors (e.g. logical, physical) leading to more complex attack scenario and consequently, lower risk.

http://en.wikipedia.org/wiki/Two-factor_authentication